Let's look at the code first
public class IntegerDemo { public static void main(String[] args) { String num = null; System.out.println( Integer.parseInt(num));// Exception java.lang.NumberFormatException System.out.println( Integer.valueOf(num));// Exception java.lang.NumberFormatException System.out.println( String.valueOf(num)); // Output null num = ""; System.out.println( Integer.parseInt(num)); // Exception java.lang.NumberFormatException System.out.println( Integer.valueOf(num)); // Exception java.lang.NumberFormatException System.out.println( String.valueOf(num)); // Empty string, nothing is output}}Let's take a look at how String.valueOf() is written
String.valueOf() can output normally when encountering null and empty strings, so no errors are thrown.
Let's take a look at how the packaging type Integer is handled
In both methods, parseInt(s, 10) is to convert the string s into a decimal int basic type, but valueOf() will be retrieved from the internal cache of [-127, 127] according to the int range (using the Entity Mode in the Design Mode)
Let’s take a look at parseInt(s, 10). In the method, we will judge whether the string is a legal number, and we will verify other formats such as null, empty string, etc., so we will throw an error
public static int parseInt(String s, int radix) throws NumberFormatException { /* * WARNING: This method may be invoked early during VM initialization * before IntegerCache is initialized. Care must be taken to not use * the valueOf method. */ if (s == null) { throw new NumberFormatException("null"); } if (radix < Character.MIN_RADIX) { throw new NumberFormatException("radix " + radix + " less than Character.MIN_RADIX"); } if (radix > Character.MAX_RADIX) { throw new NumberFormatException("radix " + radix + " greater than Character.MAX_RADIX"); } int result = 0; boolean negative = false; int i = 0, len = s.length(); int limit = -Integer.MAX_VALUE; int multmin; int digit; if (len > 0) { char firstChar = s.charAt(0); if (firstChar < '0') { // Possible leading "+" or "-" if (firstChar == '-') { negative = true; limit = Integer.MIN_VALUE; } else if (firstChar != '+') throw NumberFormatException.forInputString(s); if (len == 1) // Cannot have lone "+" or "-" throw NumberFormatException.forInputString(s); i++; } multmin = limit / radix; while (i < len) { // Accumulating negatively avoids surprises near MAX_VALUE digit = Character.digit(s.charAt(i++),radix); if (digit < 0) { throw NumberFormatException.forInputString(s); } if (result < multmin) { throw NumberFormatException.forInputString(s); } result *= radix; if (result < limit + digit) { throw NumberFormatException.forInputString(s); } result -= digit; } } else { throw NumberFormatException.forInputString(s); } return negative ? result : -result; }Summarize
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